1. Field of Invention
The field of technology of the invention is glass manufacturing, and more especially the efficient melting of raw materials in a glass furnace. More particularly, the present invention relates to a method of manufacturing glass involving a process for preheating the components of the furnace load.
2. Brief Description of the Related Art
As glass manufacturing is one of the most energy intensive industries, the improvement of thermal efficiency of furnaces is an important issue. One known technique to increase the efficiency of a glass melting furnace is the recovery of the heat contained in the flue gas. The energy recovered can be used for example to preheat the furnace load, and thus reduce the energy requirement of the process. In the container glass industry, a vertical batch and cullet preheater has been described in U.S. Pat. No. 5,526,580, as exemplified by the commercial indirect preheater from Zippe of Germany. The proceeding of 1992 Thermie Seminar in Weisbaden also contained a description of a direct preheater from Interprojekt (formerly GEA). The drawbacks of these techniques lay in the fact that they are not suitable for all glass compositions, and are not suitable for all types of raw materials fed into the preheater.
Glass manufacturing involves the preparation of the raw material mixture (or load), the feeding of the raw material mixture into a furnace, the heating of the raw material mixture, its melting and fining. The raw materials include the components that are needed to produce glass of a desired composition. The mixture of non-transformed raw materials that is ready to be molten in the furnace, is referred to as the batch, which may have been wetted, mixed, or sorted. The batch is, for example, a wet mixture of sand, soda ash, limestone, etc. Cullet refers to already molten glass, either recycled from the plant (plant cullet), recycled from other plants or industries, or from post-consumers glass sources (outside or foreign cullet). The load will refer to the material actually fed into the furnace, which may include batch, cullet or additive.
Reacting materials are defined by the fact that they may react while they undergo preheating. Pre-reactions include degasing (CO.sub.2, N.sub.2, Steam, SO.sub.2 or other gases), melting (boric acid and boric oxide melting point is as low as 185.degree. C. and 300.degree. C.), chemical reactions (calcining -decomposition of the carbonates to oxides with CO.sub.2 release- or silica and soda ash reactions to produce sodium metasilicate) and important changes in the mechanical or physical properties of the material. Reacting materials may be found in any type of material, raw material, batch, cullet or load. The generic term "material" will apply to any of the components (raw material, reacting material, batch, cullet or load) described above or a mixture of thereof.
Cullet preheating is a well known technique. It is described, for example, in U.S. Pat. No. 5,556,443 (direct preheater from Edmeston of Sweden). A batch can also be preheated, but the presence of water in the batch, and the chemical reactions that occur while the batch heats up (gases release from soda ash or limestone, steam evaporation, partial melting), turn the material into a sticky mixture when it is preheated. As it is difficult to heat up the batch by itself, it is often mixed with cullet to be preheated, with a typical minimum weight ratio of cullet of 50% for efficient preheating. The mixture of cullet and batch can go through the material preheater and be preheated to 500-550.degree. C. maximum. Beyond this temperature, the cullet will also become too sticky to sustain further heating.
U.S. Pat. No. 5,713,977 describes load preheating in order to avoid the problems of particle entrainment and load reaction. Particle entrainment is addressed by creating a filter using the load.
To ensure a good glass quality, homogeneity of the melt is required. The careful and thorough mixing of the cold batch prior to charging into a conventional furnace is a common way to ensure good homogeneity in the glass bath. It limits the material preheating of the load, however, because if the load has a low weight fraction of cullet, as for example in fiber glass production or a float glass furnace, or has a component with a low melting point, such as boron for a borosilicate glass composition, it cannot be preheated in the existing material preheaters. The current possibility of material preheating and energy savings in such glass furnaces is therefore restricted and limited.
The glass industry would be well served by a method which allowed for energy efficient preheating even when the cullet fraction of the load is small. It is therefore an object of the present invention to provide such a method.
This and other objects of the present invention will become apparent to the skilled artisan upon a review of the following description, the drawing and the claims appended thereto.